Dark energy may evolve over time, suggests largest 3D map of universe

Using data from 15 million galaxies and quasars (extremely distant yet bright objects with black holes at their cores), DESI has produced the largest-ever 3D map of the universe, providing the most precise measurements of how cosmic structures have evolved over the past 11 billion years.

These findings strengthen hints that dark energy may not be constant over time, challenging the current standard model of cosmology.

The fate of the universe depends on the balance between matter and dark energy. The simplest model suggests that dark energy is constant over time, described by Einstein’s equations in the Lambda Cold Dark Matter model.

However, when DESI’s new data is combined with other cosmic observations – including the light leftover from the dawn of the universe (the cosmic microwave background or CMB), exploding stars (supernovae), and how light from distant galaxies is warped by gravity (weak lensing) – scientists find growing evidence that dark energy’s influence may be changing over time.

DESI is an international experiment with more than 900 researchers from over 70 institutions around the world and is managed by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).

The collaboration shared its findings today on the online repository arXiv and in a presentation at the American Physical Society’s Global Physics Summit in Anaheim, California.

Professor Ofer Lahav (UCL Physics & Astronomy), a DESI collaborator and a member of its Executive Committee, said: “Tracing the evolution of the dark energy concept over the past century is fascinating. It started with Einstein adding a number to his theory of general relativity to account for a static universe. He dismissed this number, known as a cosmological constant, when it became clear the universe was expanding. Then the idea was reclaimed to represent a constant dark energy driving the universe’s accelerated expansion.

“Now, DESI observations, combined with other probes, suggest the intriguing possibility that the density of dark energy may be evolving with cosmic time. If confirmed, this would represent a paradigm shift in our understanding of the universe.

“If dark energy is constant, the universe would continue to expand at an accelerating rate forever. If it evolves with time, the fate of the universe is more uncertain.”

Dr Paul Shah (Honorary Research Associate, UCL Physics & Astronomy), a member of the Dark Energy Survey supernovae team, said: “In Einstein’s original equation, dark energy is a constant of nature that doesn’t change with time or place.

“The pattern of galaxies observed by DESI suggests this isn’t true, but on its own this isn’t strong enough evidence. However, when this is paired with observations of supernovae from the Dark Energy Survey, in which UCL has also played a key role, we can build a history of how dark energy seems to be evolving with time. Pulling together evidence from multiple surveys in this way reduces the possibility that this is chance or misinterpretation of the data.

“If it is true that dark energy evolves with time, this would be one of the most significant discoveries in cosmology in a generation, as it implies something we don’t yet understand is causing the change.”

Professor Alexie Leauthaud-Harnett, co-spokesperson for DESI based at UC Santa Cruz, said: “What we are seeing is deeply intriguing. It is exciting to think that we may be on the cusp of a major discovery about dark energy and the fundamental nature of our universe.”

Taken alone, DESI’s data is consistent with our standard model of the universe: Lambda CDM, where CDM is cold dark matter and lambda represents the simplest case of dark energy, where it acts as Einstein’s cosmological constant.

However, when paired with other measurements, there are mounting indications that the impact of dark energy may be weakening over time and other models may be a better fit.

Professor Will Percival, co-spokesperson for DESI based at the University of Waterloo, said: “We’re guided by Occam’s razor [the principle that the simplest explanation is the best], and the simplest explanation for what we see is shifting.

“It’s looking more and more like we may need to modify our standard model of cosmology to make these different datasets make sense together – and evolving dark energy seems promising.”

DESI contains 5,000 fibre-optic “eyes”, each of which can collect light from a galaxy in just 20 minutes. Professors Peter Doel and David Brooks at UCL Physics & Astronomy helped design, assemble and build the DESI’s optical corrector – six lenses, the largest 1.1m across, that focus light on to the “eyes”. The optical corrector construction was supported by STFC. 

DESI is mounted on the U.S. National Science Foundation’s Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory (a program of NSF NOIRLab) in Arizona. The experiment is now in its fourth of five years surveying the sky, with plans to measure roughly 50 million galaxies and quasars by the time the project ends.

LinkThe new analysis uses data from the first three years of observations and includes nearly 15 million of the best measured galaxies and quasars. It’s a major leap forward, improving the experiment’s precision with a dataset that is more than double what was used in DESI’s first analysis, which also hinted at an evolving dark energy.

Professor Seshadri Nadathur, of the University of Portsmouth, said: “It’s not just that the data continue to show a preference for evolving dark energy, but that the evidence is stronger now than it was.

“We’ve also performed many additional tests compared to the first year, and they’re making us confident that the results aren’t driven by some unknown effect in the data that we haven’t accounted for.”

DESI tracks dark energy’s influence by studying how matter is spread across the universe. Events in the very early universe left subtle patterns in how matter is distributed, a feature called Baryon Acoustic Oscillations (BAO).

That BAO pattern acts as a standard ruler, with its size at different times directly affected by how the universe was expanding. Measuring the ruler at different distances shows researchers the strength of dark energy throughout history. DESI’s precision with this approach is the best in the world.

Dr Willem Elbers of Durham University said: “For a couple of decades, we’ve had this standard model of cosmology that is really impressive.

“As our data are getting more and more precise, we’re finding potential cracks in the model and realising we may need something new to explain all the results together.”

The collaboration will soon begin work on additional analyses to extract even more information from the current dataset, and DESI will continue collecting data. Other experiments coming online over the next several years will also provide complementary datasets for future analyses.

DESI’s Data Release 1 (DR1) is now available for researchers worldwide, enabling further studies on galaxy evolution, black holes, and dark matter.

DESI is supported by the DOE Office of Science and by the National Energy Research Scientific Computing Center, a DOE Office of Science national user facility. Additional support for DESI is provided by the U.S. National Science Foundation; the UK’s Science and Technology Facilities Council (STFC); the Gordon and Betty Moore Foundation; the Heising-Simons Foundation; the French Alternative Energies and Atomic Energy Commission (CEA); the National Council of Humanities, Sciences, and Technologies of Mexico; the Ministry of Science and Innovation of Spain; and by the DESI member institutions.

Links

• Dark Energy Spectroscopic Instrument (DESI)
• Professor Ofer Lahav’s academic profile
• Dr Paul Shah’s academic profile
• UCL Physics & Astronomy
• UCL Mathematical & Physical Sciences
• DESI’s data release

Source

• Berkeley Lab

Images

• Main: DESI has made the largest 3D map of our universe to date and uses it to study dark energy. Earth is at the centre in this animation, and every dot is a galaxy. Credit: DESI collaboration and KPNO/NOIRLab/NSF/AURA/R. Proctor
• Embedded: DESI maps distant objects to study dark energy. The instrument is installed on the Mayall Telescope, shown here beneath star trails. Credit: KPNO/NOIRLab/NSF/AURA/B. Tafreshi

Media contact

Mark Greaves

T: +44 (0)7990 675947

E: m.greaves [at] ucl.ac.uk

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